1. Field of the Invention
This invention relates to friction compensation and, more specifically, to an adaptive friction compensator (AFC) to augment conventional stabilization rate loops to improve line-of-sight (LOS) jitter performance, primarily in LOS stabilization and pointing systems.
2. Brief Description of the Prior Art
Frequency compensation is generally utilized in conventional stabilization rate loops to improve LOS jitter performance in stabilization and pointing systems by canceling torque disturbances, predominantly due to bearing and torquer friction. Torque disturbances in the stabilization system cause LOS jitter which limits and frequently hinders the desired overall system performance, such as modulation transfer function (MTF) (a measure of the amount of degradation of a recorded image when LOS jitter is introduced), tracking, identification, range requirements, etc. These torque disturbances arise mainly from the linear and/or rotary base environments to which the systems are subjected during their mission. Common major sources of torque disturbances in linear environments are due to gimbal imbalance and structural flexure. In rotary environments, major disturbances include coulomb and viscous friction and cable/gas-line spring and damping effects. Since most environments are a combination of linear and rotary vibrations, all of the above mentioned disturbances contribute to the LOS jitter. Possibly the most difficult source of disturbances to estimate and reduce in hardware is coulomb friction in bearings, seals, twist caps and brush torquers. Friction is also difficult to control in hardware due to the following sources which cause the characteristics of friction to change with time:
a) temperature effects, which cause variations in alignment, stress and bearing pre-load.
b) non-linear characteristics of friction, which are a function of relative rate amplitude and frequency content.
c) gimbal pointing angle, due to variations in pre-load and tolerances along the bearing circumference.
d) gimbal slew rate, which affects the relative rate dynamics and introduces variable lubrication effects as the bearing balls rotate.
From a control and/or compensation point of view, coulomb friction is also one of the most difficult disturbances to reject due to its step-like disturbance characteristics. For this reason, friction disturbances require high bandwidth compensation techniques with time delays negligible compared to the rise time of the disturbance waveform. Conventional rate loops, for reasons discussed hereinbelow, are significantly limited in bandwidth for purposes of compensating coulomb friction disturbances. By the time the rate loop senses LOS jitter, such jitter has already occurred and requires a finite amount of time to correct.
One of the requirements of a well designed stabilization rate loop is the cancellation or rejection of torque disturbances by generating a torque command equal and opposite to the disturbance as it occurs. This is generally accomplished in conventional rate loops by feeding back the LOS jitter rate obtained with a gyroscope and applying proportional plus integral (PI) compensation to generate the torque commands. The main limitation in rejecting the disturbances is the time delay inherent in rate loops due to bandwidth limitations of typically 40 to 90 Hz. These limitations arise mainly from gyroscope noise which causes saturation if an attempt is made to increase the bandwidth and from gimbal structural resonances which cause the rate loop to become unstable if the bandwidth is increased significantly. The fundamental problem with disturbance cancellation using a rate loop is that LOS motion must first exist before the rate loop can become active. However, the primary goal of stabilization is to eliminate such LOS motion as caused by unknown, uncontrollable torque disturbances and not permit it to occur in the first place. Other requirements of a well designed rate loop include precise control of LOS scan and slew rates and LOS step-stare pointing.
The prior art includes an "adaptive bearing compensator" as set forth by Walrath, C. D., "Adaptive Bearing Friction Compensation Based On Recent Knowledge of Dynamic Friction", Automatica, Vol. 20, No. 6, 1984, to solve the friction torque disturbance problems discussed herein. The main disadvantage of the adaptive bearing compensator is the lack of real-time feedback to update the friction model which estimates the disturbance. Without such feedback or adaptive mechanism, there is no way to assure that all or most of the friction disturbance is actually being canceled. Moreover, there is no way to assure that the disturbance is not being over-compensated due to a decrease in friction caused, for example, by changes in temperature and/or relative rate frequency spectrum. Instead, the proportionality constant relating friction to relative rate is verified during laboratory experiments and assumed to have a constant relationship to friction during system operation.
The torque observer approach in the prior art, much like the rate loop, is limited in bandwidth due to gimbal structural resonances and acceleration sensor bandwidth and noise. In addition, it requires a mathematical model of the gimbal inertia and structure, but provides no means for adaptively updating and verifying the model in real-time. It is expected that the total gimbal inertia and frequency of structural resonances will vary with gimbal angle.
In the case of a non-linear controller, in order for this technique to yield significant improvement in stabilization, the large-error-gain must be increased significantly compared to the nominal loop gain. This increase in gain results in momentary instability when the LOS error (or jitter level) exceeds the threshold. Although stabilization is improved at the dominant frequency of the disturbance, this instability causes the LOS jitter to increase at higher frequencies in much the same manner as a limit cycle or structural resonance.
With reference to adaptive noise cancellation (ANC), the main disadvantage of this approach is the requirement of a reference signal correlated with the torque disturbance. In practice, the torque disturbance is unknown and not measurable. It is believed that the relative rate could be used as the correlated signal in ANC for friction disturbances. However, due to its linear theory development, it is not expected that the ANC will compensate the step-like non-linearities associated with friction. The computational requirements of ANC limit this technique to digital implementation in practice. Analog implementation is not feasible.